It's difficult to guess what the problem might be. Are there any errors when installing the plugin?
Maybe you could open an issue on Github with some more details like which distribution, compiler and KDE version you are using.

That part about the requirement of libqalculate v2.5 only refers to the new feature of using the two's complement representation for negative numbers. Apart from that the latest version should hopefully work with all (recent) versions of libqalculate (including v2.2).

The error message you receive looks like an ABI incompatibility between libqalculate and the plugin.
If possible please open a bug report on the Github page, which is much better suited to discuss such issues.

This looks like a version conflict. When you use libqalculate 2.0.0 it should not try to link stopPrintControl().
How did you install the libqalculate? Manually or with a package manager? What does the output of the cmake command say.
It should say something like this:
-- Checking for module 'libqalculate>=0.9.8'
-- Found libqalculate, version 2.4.0

Not significative output differences compiling with any of 2.0.0 or 2.4.0 versions of libqalculate for 'make' and 'make install' commands. { references like (.text+0x2c6a) and percent compiling advances as sole differs. ]

Usually whenever libqalculate was upgraded, all that I had to do is to recompile the plasmoid. However, after I upgraded libqalculate to 2.0 and recompiling the plasmoid, I was unable to start plasmashell; there was a "malloc(): memory corruption" message even. I was able to get plasmashell starting again after removing the plasmoid.

The moral of the story is, that plasmoid 0.3x and libqalculate 2.0 do not seem to like each other.